In the normal four-cycle internal combustion engine, a camshaft is driven at half the crankshaft rotational speed. Cams on the camshaft control the opening and closing of both the intake and exhaust valves. In the operation of a four-cycle internal combustion engine, if the cam timing and duration is such that the intake valve begins opening at top dead center of the intake stroke and is closed at the bottom center of the stroke and the exhaust valve begins opening at the bottom center of the exhaust stroke and is closed at the top center of the exhaust stroke, the engine would be restricted to low rpm operation and low power output. At high rotative speed (rpm) the breathing efficiency would be low. On the other hand, if the cam timing and duration allowed sufficient overlap of valve opening and closing at the beginning and end of both the intake and exhaust strokes, breathing efficiency would be greatly increased at higher rotative speed, although at very low rotative speed, efficiency would be impaired, economy reduced, and pollution increased.
No mechanical efficient mechanism exists which permits adjustment of the cam timing to control both the point of valve opening and the duration of valve opening of both sets of valves during engine operation. Such valve adjustment is necessary for optimizing engine function with varying conditions. The principal change in engine operating conditions, for which valve open duration and timing control is advantageous, is change in rotative speed, although other engine operating criteria such as load, fuel quality, air temperature and pressure and the like have a small effect on engine operating conditions and can be employed as signals for valve open duration and timing adjustment.